Could be to do with the sacraments they are allowed to perform.
The seven sacraments are:
Baptism
Confirmation
Eucharist
Penance
Anointing of the Sick
Holy Orders or Ordaining
Matrimony
I think anybody can baptise like a deacon for example but only bishops can perform all of them including confirming and ordaining and priests the other five .
I can find no clear reference for the "crosses on a tomb" explanation - this seems to only exist on quiz sites and as a trivia factiod.
When I researched a similar question last year I could only arrive at the same conclusion as that offered by shaneystar2, although technically in the Western Church it is Matrimony which is ministered by "ordinary" people and presided over by the clergy (though Baptism can be ministered by anybody in extraordinary circumstances).